Assembly station and management method therefor

ABSTRACT

Automated parts assembly station comprising positioning means ( 11 ) having surfaces ( 15   a   , 16   a ) defining the desired mutual position of the parts to be assembled and assembly means ( 13 ) connecting the parts. At at least some of said surfaces ( 15   a   , 16   a ) there are measurement means ( 18 ) for the distance between said at least some surfaces and facing surfaces of the parts being assembled.  
     In such a station a method for the timely detecion of assembly tolerances comprises the steps of performance with the positioning means of a permanent positioning of the parts being assembled, operation of the assembly means for connection of the parts, release of the positioning means to take them to a loose positioning condition of the parts, measurement with the measurement means of the distance between the assembled parts and said at least some surfaces, comparison of the measured distances with maximum admissible distances, and generation of an acceptance or non-acceptance signal depending on whether the distances measured are greater or less than the related maximum admissible distances.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an automated assembly stationfor mechanical parts and a management method therefor.

[0002] Automated precision assembly stations comprising a structurebearing a plurality of locking inserts for mutual positioning of theparts to be assembled are well known in the prior art. Assembly devices,for example articulate robot arms, permanently fasten together the partspositioned by the inserts and the finished product is then drawn fromthe station.

[0003] Assembly can take place by welding, gluing, riveting, clinchinget cetera depending on specific requirements. Examples of assemblystations are the welding stations on assembly lines for motor vehiclesor their parts.

[0004] Usually when the assembly line of which the station is a part ismade operational there is performed a fine calibration of the positionof the inserts which represent ideally key reference points of thestructure it is desired to assemble, that is to say points through whichthe structure must pass to be defined optimal. For this purpose theinserts are shaped with gripping surfaces (usually consisting of a fixedpart and a facing complementary movable part) which represent the idealconfiguration at those points of the object to be assembled. Thesurfaces of the inserts are generally shaped starting from themathematics of the surface of the part it is desired to secure and takenfrom the CAD drawings representing it.

[0005] Various factors concur however to make less than ideal theconfiguration of the real object produced which will be subject togeometrical errors causing its deviation from the optimal configuration.Degradation factors can be proper to the assembly technique used or dueto geometrical changes which intervene after initial calibration of thestation. For example in the case of welding assembly variations ordrifting of the welding parameters such as those due to consumption ofthe electrode, variation of the electrical current or welding time,change of position in the sequence of performance of the welding pointset cetera can intervene. Similar factors can be traced for the otherassembly methods also.

[0006] Geometrical variations can be caused for example by maintenanceoperations and/or adjustments to the station, wear of movable parts oreven variations in the physical parameters of the parts assembled suchas in the composition, thickness, shape et cetera of the parts to beassembled. Typical are the changes in thickness of pressed sheet metalparts with changes in the production lot.

[0007] For each reference point there is thus defined a tolerance rangewithin which the object produced must fall to be accepted at least as asatisfactory part.

[0008] Usually purposive measurement stations arranged at line endperform sample measurements of the parts produced and if necessary emitan unacceptability signal which warns of the need to reject the parts inproduction and seek along the line the reason or reasons which producedthe undesired change. The difficulty of identifying the responsiblestation among the plurality present in the line is apparent.

[0009] In addition, as the measurement is done on a sampling basis acertain number of defective parts are produced before the defect issignaled.

[0010] Since sequential assembly operations are generally performed onvarious stations before obtaining the final part which is measured on asampling basis errors produced in an early station and which ifdiscovered early would be correctable or limitable become irreparableand also involve members assembled in the subsequent stations. All thisresults in wasted time, money and materials.

[0011] The general purpose of the present invention is to overcome theabove shortcomings by supplying an innovative assembly station and amanagement method allowing timely and accurate discovery of unacceptabledeviations from the ideal form. Another purpose of the present inventionis to point out in a timely manner changes in the reference geometry ofthe station resulting for example from faulty maintenance or wear. Inaddition another purpose is to facilitate recalibration of the stationreference members after maintenance operations and/or worn partsreplacement.

SUMMARY OF THE INVENTION

[0012] In view of these purposes it was sought to provide in accordancewith the present invention an automated parts assembly stationcomprising positioning means having surfaces defining the desired mutualposition of the parts to be assembled and assembly means connecting theparts together characterized in that opposite at least some of saidsurfaces there are means of measurement of the distance between said atleast some surfaces and facing surfaces of the parts assembled. In sucha station a method of timely detection of assembly tolerances comprisesthe steps of making with the positioning means a permanent positioningof the parts to be assembled, operation of the assembly means forconnection of the parts, release of the positioning means to take theminto a loose positioning condition of the parts, measurement with themeasurement means of the distance between the assembled parts and saidat least some surfaces, comparison of the measured distances withadmissible maximum distances, and generation of an acceptance ornon-acceptance signal depending on whether the distances measured aregreater or less than the admissible maximum detected distances.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] To clarify the explanation of the innovative principles of thepresent invention and its advantages compared with the prior art thereis described below with the aid of the annexed drawings a possibleembodiment thereof by way of non-limiting example applying saidprinciples. In the drawings:

[0014]FIG. 1 shows a diagrammatic perspective view of an assemblystation in accordance with the present invention,

[0015]FIG. 2 shows a diagrammatic perspective view of a positioninginsert located in the station of FIG. 1,

[0016]FIG. 3 shows a partially cross-sectioned diagrammatic view of adetail of an insert of the type of FIG. 2, and

[0017]FIG. 4 is a cross section view of a contact zone of an insert.

[0018] With reference to the FIGS. FIG. 1 shows an automated partsassembly station indicated as a whole by reference number 10 comprisingreference and positioning means 11 which define the desired mutualposition of parts 12 to be assembled and assembly means 13 which connectthe parts positioned by the reference means. The parts to be assembledcan be for example automotive vehicle body members.

[0019] The assembly means can employ any known parts assembly systemsuch as riveting, welding, gluing et cetera. In the FIG are shown asexamples robot arms for welding with welding electrodes 14.

[0020] As may be seen in FIG. 2, the positioning means comprise inserts15, 16 which define positioning surfaces 15 a, 16 a shaped on themathematics of the parts to be produced. These surfaces define thepoints through which the structure being assembled must pass exactly inorder to be described as optimal.

[0021] The inserts are made up of a fixed member 15 and a facing movablemember 16. The movable member can be drawn on command near to the fixedmember by means of an actuator 17 so as to close on a part to beassembled.

[0022] During operation of the station the parts to be assembled areheld together and pushed against the positioning surfaces consisting ofthe fixed inserts 15 by the load applied by the movable inserts orlocking members 16. If after the assembly process the assembled partadheres perfectly to the positioning surface of the fixed member 15 evenafter opening of the locking member 16 the assembled part is within thedesired tolerances.

[0023] As may be seen diagrammatically in FIG. 3 in accordance with thepresent invention, at at least some of the positioning and supportsurfaces there are measuring means 18 for the distance between thesesurfaces and facing surfaces of the parts being assembled. In particularthe measuring means are contained in the fixed member 15 to appear fromits surface and comprise linear deviation detection sensors 18 arrangedvirtually perpendicular to the surface.

[0024] As may be seen in FIG. 4 in a preferred embodiment the sensors 18are deviation detectors equipped with a measuring rod 19 which runs oncommand by means of a compressed air feed 21 out of the surface of thefixed member to rest with a tracer ball 20 on the facing surface. FIG. 4shows the case of a positioning surface with complex form and equippedwith two sensors while in FIG. 3 the two positioning surfaces areseparated from each other and each one is equipped with its own sensor.

[0025] Naturally the number of sensors and their exact arrangement willdepend on the configuration of the support surface and on thepositioning errors it is desired to detect.

[0026] In using the station a stable position of the parts to beassembled is made first using the reference means 11. Then the assemblymeans for connection of the parts (for example by welding) are operated.Then the positioning means are released and brought into a loosepositioning condition of the parts that is to say with the parts restingon the reference surfaces of the fixed inserts but free of therestrictions imposed by the movable inserts. The completed assembly canthus take on its own free configuration. At this point the measurementmeans are operated to measure the distance between the referencesurfaces and the facing surfaces of the assembled parts. For definitionof the reference surfaces, if the assembled parts touched all thereference surfaces the product obtained would be perfectly in the centerof the tolerances and the distances measured would all be zero.

[0027] The distances measured are compared by a comparison device 22with maximum admissible distanced memorized in a memory 23. Thecomparison device 22 generates an acceptance or non-acceptance signal 24depending on whether the distances measured are greater or smaller thanthe admissible maximum distances.

[0028] In this manner assembled units which do not meet thespecifications can be immediately detected and rejected. In addition thestation in which the defect is found is immediately detected.

[0029] With the station in accordance with the present invention it isalso possible to detect defects in parts before assembly in addition toverifying the geometry of the composite object.

[0030] In the use of the station it is also possible before operation ofthe assembly means to perform loose positioning of the parts to beassembled with respect to the positioning means. That is to say placethe parts on the surfaces without closing the locking inserts 16 andmeasure with the measuring means the distance between the partspositioned and the positioning surfaces. The measured distances are thencompared by the device 22 with admissible maximum distances memorized inthe memory 23 and an acceptance or non-acceptance signal 24 is generateddepending on whether the measured distances are greater or smaller thanthe related admissible maximum distances. Closing of the inserts forpermanent positioning of the parts and operation of the assembly meansare performed only in case of acceptance.

[0031] This avoids performance of an assembly which would give anunacceptable result. In addition in case the geometry defect is due onlyto some parts it is possible to recover the other parts.

[0032] Another advantage of a station in accordance with the presentinvention is that it is possible to detect defects in the station'spositioning means. To secure this in using the station it is possible tomeasure with the measuring means the distance between the positionedparts and the corresponding positioning surfaces during permanentpositioning of the parts and compare the distances measured with maximumadmissible distances memorized in the memory 23. The device 22 thengenerates a conformity or non-conformity signal 24 for the stationdepending on whether the measured distances are greater or smaller thanthe related maximum admissible distances.

[0033] As another verification of the correctness in time of thepositioning means if the fixed and movable surfaces are facing it isalso possible before the pieces to be assembled reach the station tomake with the measurement means a measurement of the distance betweenthe fixed surfaces and facing movable clamping surfaces, and compare thedistances measured with distances memorized previously in the memory 23and defined as optimal distances corresponding to a perfectly efficientstation. The device 22 emits a non-conformity signal 24 for the stationif the result of the comparison indicates deviation of the measureddistance from the memorized distance greater than a tolerance which wasdefined as acceptable in advance.

[0034] In this manner erroneous calibration of the movable part withrespect to the fixed part caused for example by poor maintenance or playproduced by use is identified early. In the case of maintenanceinvolving the movable parts of the inserts it is also possible to usethe signal 24 to repeat adjustment of the movable parts with respect tothe fixed parts. This makes any maintenance of the movable parts of thestation much faster and easier.

[0035] To secure distances to be memorized in the memory 23 forcomparison purposes it is possible to use the same measurement means ofthe station. To do this it is sufficient to adjust the position of thepositioning means to have the surfaces thereof define an optimalposition of sample parts to be assembled. Such a procedure is usuallyperformed during installation and initial calibration of a station. Thestation is then commanded to move every movable member to its closingposition. The measurement means are then commanded to measure thedistance between the surface of the fixed member and the facing surfaceof the movable member. As the measurement is performed on a stationdetermined to be efficient these measured distances are memorized in thememory 23 as optimal distances.

[0036] It is now clear that the preset purposes have been achieved bymaking available a station and management method permitting detection inreal time of assembly defects and obtaining not only assembled memberswhich meet precise dimensional specifications but also detect defects inthe assembly station itself while providing guidance in repair andmaintenance thereof.

[0037] Naturally the above description of an embodiment applying theinnovative principles of the present invention is given by way ofnon-limiting example of said principles within the scope of theexclusive right claimed here.

[0038] For example both the form and arrangement of the positioning andassembly means and the operation sequence of the station can changedepending on specific requirements. In addition the distance measurementsensors can be of any suitable type including optical, inductive,capacitative et cetera. For calibration of the station there can also beprovided purposive shims to be placed between the fixed and movableparts of the inserts to identify a reference surface facing and possiblyadhering to the surface of the locking insert bearing the sensor.

[0039] With a station in accordance with the present invention it ispossible to verify the efficiency of the station itself merely byplacing therein a sample part and then checking whether the deviationsignals produced by the sensors of the station are the same as thosewhich an efficient station should detect on that sample part unlessthere is a deviation considered acceptable.

[0040] In addition to verification of station efficiency the sample partcan also be used for its calibration whether initial or for maintenance.

[0041] The sample part can be either a part which has been determined tobe accurate that is to say a part with tolerances determined in advanceas acceptable with respect to the ideal part or a part which althoughnot acceptable has known deviations with respect to an ideal piece. Inother words the sample can be either a certified acceptable sample suchas the so-called ‘golden part’ or a part which has been measured on aknown measurement station and whose deviations from the ideal—orgolden—part are known. The second case has been found particularlyadvantageous because it allows use of a not particularly valuable partwhich could even be a part rejected because out of tolerance.

What is claimed is:
 1. Automated parts assembly station comprisingpositioning means having surfaces defining the desired mutual positionof the parts to be assembled and assembly means connecting the partscharacterized in that at at least some of said surfaces there aremeasurement means for the distance between said at least some surfacesand facing surfaces of the parts being assembled.
 2. Station inaccordance with claim 1 characterized in that the positioning meanscomprise reference inserts each made up of a fixed member and a facingmovable member with the movable member being capable of approaching thefixed member on command to close on a part to be assembled.
 3. Stationin accordance with claim 2 characterized in that the measurement meansare contained in the fixed member to appear from its surface.
 4. Stationin accordance with claim 1 characterized in that the measurement meansare linear deviation detection sensors virtually perpendicular to thesurface of the positioning means.
 5. Station in accordance with claim 4characterized in that the deviation detectors have a measuring rodsliding on command out of said surface to move from a non-operationalposition to an operational or measurement position.
 6. Station inaccordance with claim 4 characterized in that the assembly means arerobotized welding arms.
 7. In an automated parts assembly stationcomprising positioning means having surfaces defining the desired mutualposition of the parts to be assembled and assembly means connecting theparts with there being at at least some of said surfaces measurementmeans for measurement of the distance between said at least somesurfaces and facing surfaces of the parts being assembled a method forthe timely detection of assembly tolerances comprises the steps of:performance with the positioning means of a permanent positioning of theparts being assembled, operation of the assembly means for connection ofthe parts, release of the positioning means to take them to a loosepositioning condition of the parts, measurement with the measurementmeans of the distance between the assembled parts and said at least somesurfaces, comparison of the measured distances with maximum admissibledistances, generation of an acceptance or non-acceptance signaldepending on whether the distances measured are greater or less than therelated maximum admissible distances.
 8. Method in accordance with claim7 comprising the additional steps of: performance before assembly meansoperation of a loose positioning of the parts to be assembled withrespect to the positioning means, measurement with the measurement meansof the distance between the positioned parts and said at least somesurfaces, and comparison of the distances measured with maximumadmissible distances, generation of an acceptance or non-acceptancesignal depending on whether the distances measured are greater or lessthan the related maximum admissible distances, and operation of theassembly means only in case of acceptance.
 9. Method in accordance withclaim 7 comprising the additional steps of: measurement with themeasurement means of the distance between the parts positioned and saidat least some surfaces during permanent positioning of the parts, andcomparison or the measured distances with maximum admissible distances,generation of a conformity or non-conformity signal for the stationdepending on whether the measured distances are greater or less than therelated maximum admissible distances.
 10. In a station such as that ofclaim 7 and with the positioning means comprising reference inserts eachmade up of a fixed member and a facing movable member with the movablemember being approachable on command to the fixed member to close on apart to be assembled and with the method comprising the additional stepsof: measurement with the measurement means before the parts to beassembled reach the station of the distance between said at least somesurfaces and facing movable clamping surfaces with which the positioningmeans are equipped, comparison of the measured distances with distancesmemorized previously and defined as optimal distances, and sending of anon-conformity signal for the station if the result of the comparisonindicates a deviation of the measured distance from the memorizeddistances greater than a tolerance previously determined to beacceptable.
 11. Method in accordance with claim 10 comprising theadditional steps of: regulation of the position of the positioning meansto provide that their surfaces define an optimal position of sampleparts to be assembled, shifting of each movable member to its closingposition, measurement with the measurement means of the distance betweenthe surface of the fixed member and the facing surface of the movablemember, and memorization of the measured distances as distances definedoptimal.